Imagine this, you have more than two networks separated by Virtual Lans protocols (IEEE 802.1q) or VLANs, carried to you by an intelligent/manageable switch on one troncal line 10/100/1000 MB HD/FD (naturally the best is 1000 MB FD).

This page introduce how to create High Performance Firewall / Nat with iptables and VLANs and iproute2. Then you can share internet to a really BIG numbers of hosts, and maintain a good performance.

Contents

VLAN support

The round robin NAT

Let's suppose we have a one ip: 200.aaa.bbb.6 and our gateway is 200.aaa.bbb.1. we can safely put these parameters by default in our configuration. It will not get participation at all in our firewall.

I say I have 3 groups of 10 IPs each to play...... we'll define the NEXT in our firewall script:

The High Performance

In our run to get a really big number of hosts running through our machine we miss some things

We forget that is just one NICs to potentially more than 8000 Mac Addresses. The card shared memory is not prepare for this!!!!!

By default iptables is not prepared to make this number of connections simultaneously !!!!!!

So...

To the first issue...
I get some error messages in the logs relative to this, I'm really sorry, I lost these logs and do not remember what they said. But the answer is this, increase the threshold memory to the neighbours.
Type this and read:

and make sysctl -p to increase to the double!!! (no reboot needed) with this I get no errors!!!!!

The next part will need some comprehension about buckets and conntracks and hashsize (the way how iptables manage the nat connections).
There is a very good document about this at here. Read it!!!!
Some thing are change since IPtables is know as Netfiler.

The last ones is just to avoid some problems that we have with ftp connections (I thing this is not necessary anymore).
The 'nf_conntrack hashsize=1048576' increase the numbers of the hashsize (increase the kernel memory designated to NAT connections) (need reboot or reload module :-) see with dmesg | grep conntrack)

And the next is put some similar to the /etc/sysctl.d/99-sysctl.conf: file

...
net.netfilter.nf_conntrack_max = 1048576
...

And do the sysctl --system command

In my case is the same number, that means that I have 1 connection for bucket!!!! I do not need more!!!! by default NetFilter put rate of 1:8. I.E. 8 conections per bucket!! (I think, not remember well)..

In our case we get about 600.000 simultaneous connections in 2 1Giga NICs cards, You can see this with the next command

# cat /proc/sys/net/netfilter/nf_conntrack_count

And put this in a snmpd agent to get and graph it in a MRTG/cacti server ..... uuuuuuu homework

The iproute2

We have 3 big access to Internet!!! This is because we manage 3 class C groups of IPs (some restrictions of BGP) in this firewall. So, we have 3 incoming traffics that we can manage, but only one outgoing!!! Our default gateway.
This can easily fill our outgoing quote, so we have to spare it.

It's recommended but not necessary put the local interfaces to each table. If you do not put the next few lines you will get not answer of ping in the local network, but you will be able to pass trough.